The article provides an overview of the methods used for processing of red mud to extract rare earth elements (REEs). Red mud is a toxic and highly alkaline waste. Several methods have been adopted and been practiced all over the world for the processing of red mud. Complex processing of red mud is cost-effective since red mud contains elements such as iron, aluminum, titanium, calcium, and rare earth metals. It has been observed that the acid leaching of red mud can almost completely recover the rare earth elements in the solution with various individual techniques and also a combination of them. Therefore, the choice of extraction method depends on the form in which the element occurs in the solution. However, relatively low concentrations of rare earth in the solution and significant amounts of impurities increase the cost of getting the final commercial products. To ensure the cost-effectiveness of the process involving rare earth’s extraction from red mud, it is necessary to increase their content by several times. This article presents the various studies that have been carried out in these aspects and the possibility of making this resource a sustainable one for REE extraction with a special focus on scandium replenishment.

The extraction of rare earth elements (REEs) from mineral resources is of significant global importance due to their critical role in modern technologies. This study focuses on the leaching behavior of artificial cerium and lanthanum phosphates using nitric, sulfuric, and hydrochloric acids under varying conditions of acid concentration and temperature. Our experiments demonstrated that the maximum extraction efficiency of lanthanum and cerium was achieved with nitric acid solutions at concentrations of 12.5 mol/dm³, with 85.0% and 79.1% extraction efficiency, respectively. The leaching mechanism involved the protonation of phosphate anions, which disrupted the phosphate matrix, facilitating the dissolution of lanthanum and cerium into solution. Sulfuric acid, although less effective at room temperature, proved to be the most thermodynamically favorable leaching agent at higher temperatures due to the formation of stable sulfate complexes. Additionally, hydrochloric acid displayed high selectivity for cerium extraction, although its applicability is limited by complexation and environmental concerns. This study offers new insights into optimizing REE recovery from phosphates, demonstrating the advantages of sulfuric acid for industrial-scale leaching processes due to its economic and thermodynamic benefits. The novelty of this work lies in its systematic comparison of the three acids’ effects on REE extraction, providing a comprehensive framework for selecting optimal leaching agents based on specific operational conditions.

The micro-arc oxidation (MAO) technique was employed to produce calcium phosphate coatings on titanium surfaces using an electrolyte composed of hydroxyapatite and calcium carbonate in an aqueous solution of orthophosphoric acid. The coatings’ morphology and composition were regulated by adjusting electrical parameters, specifically the duty cycle and voltage. This study examined the effects of the duty cycle and voltage during the MAO process on the microstructure and composition of calcium phosphate coatings on VT1–0 titanium substrates. Scanning electron microscopy (SEM) was utilized to analyze the microstructure and thickness of the coatings, while X-ray diffraction (XRD) was employed to determine their phase composition. The findings reveal that the surface morphology of the calcium phosphate coatings transitions from a porous, sponge-like structure to flower-like formations as the duty cycle and voltage increase. A linear increase in the voltage within the applied duty cycles led to a rise in the size of the forming particles of amorphous/crystalline structures containing phases of monetite (CaPO₃(OH)), monocalcium phosphate monohydrate (Ca(H₂PO₄)₂·H₂O), and calcium pyrophosphate (γ–Ca₂P₂O₇).

The development of promising biocompatible composites based on hydroxyapatite with a metallic component is of great interest to researchers. This article describes the synthesis of hydroxyapatite powder by the hydrolytic method and presents the results of mechanical grinding of hydroxyapatite powder. Additionally, in order to study the interaction between titanium and hydroxyapatite powders, the results of their thermal treatment in the temperature range of 600–900 °C are presented. As a result of the hydrolytic method, a powder consisting of Ca₅(PO₄)₃(OH) and CaO phases with a fraction of 400–600 μm was obtained. According to the results of mechanical grinding, it was determined that with an increase in grinding time from 30 to 120 min, the intensive main diffraction lines corresponding to hydroxyapatite decrease. During the thermal treatment of titanium and hydroxyapatite powders, titanium oxidizes forming suboxides and titanium dioxide (TiO₂). At higher temperatures, the hydroxyapatite phase disappears from the mixture, and titanium oxide, calcium phosphate compound, and small amounts of calcium titanate and titanium hydrophosphate are present.